As used in this application, the term "metal detector" is intended to mean metal detectors of the type used by individuals to locate buried metal objects, particularly valuable metal objects. Such metal detectors are to be contrasted with other types of metal detectors, such as military metal detectors used to locate mines, commercial metal detectors used to locate pipes and other large metal objects, and security metal detectors used in airports and other areas to locate lethal weapons (such as knives and guns). While some of the principles involved in the present invention may be useful in these other types of metal detectors, it is to be kept in mind that the invention was developed for use with metal detectors used to locate valuable metal objects buried in the ground and is expected to find its main use in such detectors.
In the past, two major types of valuable metal detectors have been marketed--beat frequency oscillator and induction-balance metal detectors. The present invention is directed to induction-balance type metal detectors, which are sometimes also referred to as transmit-receive (T/R) metal detectors.
Induction-balance metal detectors include transmit and receive coils, plus other related coils (such as feedback and null coils) mounted in a detector head. Preferably, the main coils, i.e., the transmit, receive and feedback coils, are coaxially mounted. In some detectors these coils are coplanar, as well as coaxial. In any event, the transmit coil is driven by an oscillator and the coils are formed, sized, and positioned such that the receive coil produces substantially no output signal in the absence of a metallic object (including mineralized ground) disrupting the magnetic field produced by the transmit coil. Metallic objects of both a ferrous and nonferrous nature disrupt the magnetic field produced by the transmit coil; however, in different manners. In the case of ferrous objects, the magnetic field is concentrated by the ferrous objects. In the case of nonferrous objects, eddy currents are produced in the objects that, in turn, produce magnetic fields. The eddy current produced magnetic fields dissipate the magnetic field produced by the transmit coil, in the region of the object. In either case, the magnetic field produced by the transmit coil is disrupted in a manner that generates a voltage in the receive coil. If the ground is mineralized, it generates a voltage in the receive coil having some similarity to that produced by a ferrous object. (while the receive coil voltage is similar in some respects, it is dissimilar in others. For example, a ferrous object creates a voltage pulse when the detector head passes over the object whereas the ground-produced voltage is relatively uniform.)
Induction-balance or T/R metal detectors have evolved dramatically over the past couple of decades. A brief discussion of the evolution of induction-balance or T/R metal detectors is described in the introductory portion of U.S. Pat. No. 4,507,612, entitled "Metal Detector Systems for Identifying Targets in Mineralized Ground" by George C. Payne. U. S. Pat. No. 4,507,612 describes a significant advance in the metal detector art, namely an induction-balance metal detector having the ability to specifically identify a particular type of metallic target while the target remains buried in the ground, including buried in mineralized ground. Prior metal detectors could only distinguish between ferrous and nonferrous targets. They could not identify the specific type of ferrous or nonferrous target.
While metal detectors of the type described in U.S. Pat. No. 4,507,612 have been eminently successful in identifying the nature of metal targets based on the production of an X/R quotient signal whose value is related to the phase angle of the target, they have certain disadvantages. For example, they are more complicated and, therefore, more expensive to produce than desirable. That is, the electronic circuitry described in U.S. Pat. No. 4,507,612 for producing an X/R quotient signal is relatively sophisticated. Because the circuitry is sophisticated, i.e., complex, embodiments of the invention described in this patent are more expensive to produce than desirable. Further, the target X/R quotient signal produced by metal detectors of the type described in U.S. Pat. No. 4,507,612 are related to the phase angle of targets in a nonlinear manner. As a result, a nonlinear amplifier is needed to space the readings properly on a visual display.
One approach to overcoming the foregoing and other disadvantages of metal detectors of the type described in U.S. Pat. No. 4,507,612 is described in U.S. patent application Ser. No. 556,379 filed Nov. 30, 1983 and entitled "Audible and Visual Target-Identifying Metal Detector" by Richard E. Hirschi. While the invention described in U.S. patent application Ser. No. 556,379 overcomes some of the disadvantages of the metal detector described in U.S. Pat. No. 4,507,612, some disadvantages remain. For example, while the metal detector described in U.S. patent application Ser. No. 556,379 is substantially less complicated than the metal detector described in U.S. Pat. No. 4,507,612, it is still more complicated than desirable. Further, the metal detector described in U.S. patent application Ser. No. 556,379 lacks a number of desirable features. For example, it does not have the ability to detect a target falling within a range of targets to be discriminated against. In this regard, the phase angle of some desirable targets, such as nickels, fall within the range of targets that are normally considered undesirable and, thus, discriminated against, such as nails and other ferrous objects, pull tabs, and aluminum foil. It would be desirable to provide a metal detector having the ability to selectively detect desirable targets, such as nickels, that fall within the range of undesirable targets. Another disadvantage of prior target identifying metal detectors relates to their inability to locate deeply buried targets in the presence of shallow buried targets becaue of the high magnitude signals created by shallow targets electronically hide the lower magnitude signals created by deep targets. In this regard, it should be noted that deep targets are more likely to be valuable than shallow targets, which are more likely to be nails, pull tabs and foil. Thus, it would be desirable to provide a metal detector having the ability to ignore signals created by shallow targets and respond to signals created by deep targets. The present invention is directed to providing a metal detector having the foregoing and other features and advantages.